lines on the sides or base of the tip, giving zones of high toxicity. Regrad-

292

C. BLOOMFIELD AND J. K. COULTER

ing to provide level or gentle slopes may also expose new zones of

pyrite.

B. CORROSION

Corrosion from acid sulfate waters is a common problem in mining;

the corrosion of concrete in acid sulfate soils has been described by van

Holst and Westerveld (1973); sulfate corroded the surface of concrete

piles, giving soft and porous surfaces. Similar conditions are likely to arise

with the engineering structures of irrigation projects in acid sulfate soils

in the tropics. The solution is probably the use of sulfate-resistant blastfurnace cement, or bituminous coatings on concrete.

V.

Classification and Mapping

A.

CLASSIFICATION

It has long been recognized that there are considerable variations in the

agricultural potential of acid sulfate soils, but until recently attempts at their

classification have been based largely on the amount of sulfide sulfur or

the acidity that develops on drying. Generally pH values of air-dry material

below 4.0, commonly about 3.5 have been taken as the values below which

soils may be regarded as acid sulfate soils.

Recently the importance of these soils have been recognized in classification studies (Soil Taxonomy, 1970), and a distinction has been made

between horizons that have already developed acidity i.e., acid sulfate soils,

and those that have the potential to do so, i.e., potential acid sulfate soils.

A sulfuric horizon is defined as “a mineral or organic horizon with a pH

lower than 3.5 (1 : 1 in water) and with yellow jarosite mottles with hues

2.5 or yellower and chromas 6 or more in the Munsell notation.” Sulfidic

materials are defined as waterlogged mineral or organic soil materials that

have 0.75% (on a dry weight basis) or more total sulfur, mostly in the

form of sulfides, and less than three times 2s much carbonates (CaC03

equivalent) as sulfur.

Another international definition is that in the legend of the Soil Map

of the World (Dudal, 1968) where a thionic unit is defined as one containing sufficient sulfur compounds to cause acidification of the soil, when oxidized, to a pH (in KCl) of less than 3.5,within 100 cm of the surface.

The term “pseudo-acid sulfate soil” occurs in the literature; although

no definitive terms have been given, it seems to refer to a soil containing

jarosite, but with a pH in water above 4.0, a good structure, and insufficient acidity in the form of exchangeable aluminum to prevent good crop

growth.

GENESIS AND MANAGEMENT OF ACID SULFATE SOILS

293

In the classification proposed in “Soil Taxonomy,” the great soil group

of sulfaquepts has been introduced. This comprises strongly acid soils with

a sulfuric horizon with yellow jarosite mottles and a pH (1 : 1 in water)

less than 3.5 in some part of the top 50 cm of the profile; where the acidic

horizon is between 50 and 150 cm in depth, sulfic subgroups of other

aquepts are distinguished. These would be regarded as buried cat clays.

Thus many of the acid sulfate soils of the tropics would be classified as

Typic Sulfuquepts or SuZfic Tropuquepts (van der Kevie, 1973). In the

temperate areas they would be mainly Typic Sulfaquepts and SuZfic

Haplaquepts.

Acid sulfate soils may be buried by layers of peat or muck, in which

case they may be Histic Sulfuquepts or Histic Sulfic Tropaquepts. Sulfohemists are organic soils with a sulfuric horizon within 50 cm of the surface. Potential acid sulfate soils, i.e., those with sulfidic horizons, are classified as Typic Sulfaquents if the pH (1 :1 water) of the dried soil is less

than 3.5 in the upper part of the profile. Potential acid sulfate soils that

are dominantly organic are classified as Sulfihemists. Very soft, unripened,

swamp soils that gave a pH upon drying below 4.5 in the upper 25 cm

or more may be classified as Sulfic Hydraquents. Sulfic Tropaquents and

Sulfic Fluvuquents are similar soils but with some profile development. Van

der Kevie (1973) drew attention to sulfidic soils that may be saline under

natural conditions and suggested hulk subgroups for these. Similarly some

of the sulfuquepts may also be saline where they occur in swamps, subject

to very infrequent flooding with saline water, but with a strong dry season

leading to oxidation of sulfides.

Table I, from van der Kevie (1973), summarizes the properties and

main classes as they are placed in the Soil Survey of the U.S.(Soil Taxonomy) and in the legend of the Soil Map of the World.

B. MAPPING

Potential acid sulfate areas are swampy and usually difficult of access

and often covered in heavy vegetation, either mangrove or other swamptolerant trees, or extremely tough tall grasses in the tropics, and reeds in

temperate areas. Apart from drying the samples and determining the acidity,

or other laboratory methods, including microscopic examination, there are

no completely positive ways of identifying these soils, or at least of determining how acid they will become on drainage. Several morphological features are indicative; soft muds with buried organic matter, particularly if

the remnants can be identified as halophytic, dark gray rather than greenish

gray colors, yellow mottles of jarosite in spoil from ditches, etc., are all

that meets acidity requirements for Sulfic Hydraquents but not for Thionic

Fluvisols

Not sufficiently acid for Thionic Fluvisols but meets

requirements for Sulfic

Hydraquents; nonsaline

As above

As above

hU

4

?

8

s

Worldwide

Typic Sulfihemists

Worldwide

Typic Sulfaquepts

Wet tropics and monsoon

Sulfic Tropaquepts

Acid sulfate soils

pH ( 1 : l water) <3.5 within

50 cm

pH (1:1 water) <3.5 within

50 cm

pH ( 1 : l water) <4.0 between 50 and 150 cm

and/or 3.5 to 4.0 within

50 cm

Thionic fluvisols

As above

Thionic fluvisols

As above

Thionic fluvisols

Humic gleysols

Dystric gleysols

As above

pH(KC1) 2 3.5; umbric or

0 horizon

pH(KC1) 2 3.5; ochric A

Wet tropics

Histic Sulfic Tropaquepts

Acidity as in Sulfic Tropaquepts; Histic epipedon

Thionic fluvisols

Humic gleysols

Monsoon and semiarid

Vertic Sulfic Tropaquepts

Acidity as in Sulfic Tropaquepts; cracks (1 cm)

a t 50 cm when dry

Thionic fluvisols

Humic gleysols

Dystric gleysols

Acidity as in Sulfic

Tropaquepts; Umbric or

Histic epipedon

Acidity as in Sulfic

Tropaquepts; Ochric

epipedon

Acidity as in Sulfic Tropaquepts; sodium saturation > 15 in half or more

of upper 50 cm; mostly

saline in some part of t h e

year

Thionic fluvisols

Humic gleysols

Humid temperate

Sulfic Humaqueptsb

Temperate

Sulfic Haplaquepts

Monsoon and semiarid

a

Sulfic Halaqueptsb

From van der Kevie (1973).

a Tentative subgroups suggested by van der Kevie (1973).

I value: an estimate of the degree of ripening (development).

Thionic fluvisols

Dystric gleysols

Thionic fluvisols

Gleyic solonchaks

horizon

As above

Umbric or 0 horizon

pH(KC1) 2 3.5; 0 horizon

As above

pH(KC1) 2 3.5; umbric or

0 horizon

pH(KC1) 2 3.5;ochric A

horizon

As above

pH(KC1) 2 3.5;umbric or

0 horizon

As above

pH(KC1) 2 3.5; ochric A

horizon

As above

pH(KC1) 2 3.5;salic horizon and/or saline at

some time of the year

n

m

5

*z

zm

U

5

z

Em

5

cU2

2

r

T

*4I

m

m

0,

rl

t3

W

v1

296

C. BLOOMFIELD AND J. K. COULTER

and vegetation type can have strong local correlations with sulfidic materials, and are useful in mapping on aerial photographs, on which the different mangrove species, sedges, gelam (Melaleuca) are easy to identify.

A complicating feature for soil mapping is the very strong macro- and

microvariation in levels of sulfur. Microvariation arises because of the nature of pyrite formation related to the uneven distribution of organic matter; sampling should be replicated unless large samples are taken. Macrovariation arises from the relationship between physiographic features and

pyrite formation. The degree of variation is illustrated by Westerveld and

maps on a scale of 1:25,000 to 1 :10,000 in the more homogeneous areas

and a scale as large as 1 :500 in the most heterogeneous areas. Because

of the environment it is logistically impossible to map potential acid sulfate

soils at such scales in the tropics. However the decision on whether to

develop them for agriculture usually rests more on the percentage of the

total area which will be affected by severe acidity rather than the actual

location of the areas affected. The former can be decided by adequate sampling; because of difficulties of access random sampling is usually impossible, so that some form of grid sampling is necessary; a random grid,

i.e., one in which the spacing of the grid lines is determined but the direction of the lines is decided at random, is possibly the best solution. Having

obtained an estimate of the area likely to be affected by strong acidity,

it can be decided whether the development project can support a limited

area of very infertile soils that require expensive ameliorative measures,

or whether the area affected is large enough to have a serious detrimental

effect on the agricultural prospects of the whole project.

Mapping soils that have already developed acid sulfate characteristics

is less difficult, for jarosite mottles are usually evident, and pH values can

be measured by a portable meter; agricultural development may have

cleared the area, so that the state of crop growth can be ascertained, and

the physiographical features can be clearly seen on the ground, or on aerial

photographs. However, the problems presented are usually not so much

those of identification of acid sulfate soils, but rather of assessing the seriousness of acidity, the limitations on agriculture and the probability of improvement. None of the soil classifications so far proposed can be related

directly to a land capability classification of this sort.

VI.

Conditions for Plant Growth

We have seen that acid sulfate soils are characterized by low pH values,

whereas potential acid sulfate soils have pH values near neutrality but are

GENESIS AND MANAGEMENT OF ACID SULFATE SOILS

297

strongly reduced. Rorison (1973) has listed the factors detrimental to

plant growth under strong acid conditions. These include:

1 . Direct effect due to injury by H+ions

2. Direct effect due to low pH

( a ) Impaired absorption of calcium and nitrogen

(b) Increased solubility, and thus toxicity, of iron, aluminum and

manganese

(c) Decreased availability of phosphorus, caused by aluminumphosphorus interactions, and of molybdenum

3. Low base status, leading to deficiency of calcium, magnesium, and

potassium

4. Abnormal biotic factors, such as impairment of the nitrogen cycle

and mycorrhizal activity, and increased virulence of pathogens.

Most of these have been studied on acid soils, but a few on very acid soils.

In addition, the presence of aluminum salts could lead to salt injury.